Curing of epoxies by amines at ambient and elevated temperatures is well known in the coating and adhesive arts. In structural bonding, most two-part epoxies are known to lack immediate bond strength; clamps and other fixing devices must be used to hold laminates until cured. In addition, many cured epoxies are known to be quite brittle. To toughen the epoxy matrix, many rubbery materials, such as carboxyl-terminated or acrylate-terminated butyl nitrile elastomers, have been incorporated. Elevated temperatures are usually needed for a toughening reaction to take place with these elastomer-modified materials. To achieve both film toughness and immediate bond strength through room temperature cure, the present invention describes the modification of a curable rubbery material by adding, as minor components, amine and epoxies.
U.S. Pat. No. 3,905,931 describes a curable, aqueous emulsion adhesive composition where a carboxyl-functional copolymer was blended with an epoxy resin to achieve water-resistant adhesive bonding. The '931 patent specifically describes a blend of an aqueous emulsion of poly(ethyl acrylate), an aqueous emulsion of ethyleneacrylic acid copolymer and epoxy resin. Similarly, U.S. Pat. No. 4,029,620 describes a thermally curable aqueous emulsion coating composition based on a blend of carboxylfunctional copolymer, amine, and epoxy resin. And in U.S. Pat. No. 4,342,843 a heat and water resistant latex adhesive is described based on a blend of an acid-containing neoprene latex, a polyamine and epoxy resin. However, it is well known in the art that the rate of curing acid-functional groups by epoxies is slow in aqueous environment; elevated temperatures are often needed to accelerate curing. The compositions of the present invention contain a room temperature-curable polymer latex modified with amine and epoxy additives. In contrast to the prior art, these compositions cure readily at room temperature in a few days. These compositions are prepared by mixing (1) an acidic first component containing an aqueous dispersion of polymer latex, said polymer being alkali-curable and polymerized from a monomer mixture containing quaternary ammonium monomer, and an epoxy resin, together with (2) an alkaline second component containing an amine curing reagent. The resulting mixture is alkaline, cures quickly at room temperature, and yields films with high film strength, durability, and excellent heat and water resistance.
U.S. Pat. Nos. 3,678,098 and 3,702,799 describe latex polymers containing room-temperature-curable quaternary ammonium halide functional groups. EPC Publication No. 0072158 further discloses quaternary ammonium functional monomers having surfactant anions ("liposalts") and aqueous dispersions of latex polymers prepared therefrom. The quaternary ammonium functional groups are stable at acidic pH's. They become self-reactive at alkaline pH and the polymers containing these functional groups crosslink themselves at room temperature. However, due to the polymeric nature of the material and the use of less effective simple bases, the quaternary ammonium group-containing latex polymer cures relatively slow at room temperature and the final degree of crosslinking is extremely limited. In contrast, the compositions of the present invention incorporate the more effective amines and epoxies into the quaternary ammonium group-containing latex polymers. The novel polymer mixtures of this invention cure readily at room temperature and generate much higher film strength, heat and water resistance than the alkaline-curable polymer latices described in U.S. Pat. No. 3,702,799.